All the manufacturers make excellent top end kits with heads, manifold, and cam profiles matched to what their heads flow. Consult with them, NOT Summit, Jegs, and the other mail order houses. Know this about the different cam files in a family of cams: they all make just about the same amount of power, it is just where the power band is moved to. You can see this for yourself on the CompCam website where they show the power graphs for the XE Cams. IF you use vortec heads, the XE 268 will get you your 400 hp. If you don't want to use the aggressive cam lobes of the XE cams (some people say they are too rough on the valve train) go to the Magnum series of cams from Comp or the equivalent from Isky, Crane, Howards, etc.
You really need to know what your compression ratio is and where and how this sweet little car is going to be driven. The rest will fall in place like dominoes.